1. Field of the invention
The present invention relates to a common motor such as a synchronous motor of permanent magnet type, a synchronous reluctance motor, and an induction motor, and more particularly to technology for reducing torque ripple of such a motor.
2. Description of the Related Art
Many types of motors are now widely used for various industrial and consumer uses.
A motor shown in FIG. 17 is an example of a permanent magnet synchronous motor used in a servomechanism for controlling position, speed, or the like. Thirty-six slots numbered 1 to 36 as indicated are arranged on a stator 12. A line drawn at the midpoint of each of the slots represents a boundary between windings when two sets of windings are placed within each of the slots. On a rotor side, numeral 3 denotes a rotor axis, numeral 10 a rotor core, and numeral 11 a permanent magnet. Letters N and S described on the permanent magnets indicate magnetic north and south poles, respectively. In the example shown in this figure, the rotor is a six-pole rotor wherein a 1-magnetic-pole pitch is 60 degrees as shown in the figure. Such three-phase six-pole windings as depicted in a winding diagram of FIG. 2 are looped through each of the slots of the stator 12. The labels at the top of FIG. 2 are slot indicating numbers. Three sets of winding patterns from the slot 1 to the slot 12 are placed around the perimeter of the stator. U, V and W are alternating-current terminals and N is a neutral point when a star connection is applied. In FIG. 2, windings in each of the slots of only one-third of the entire model are illustrated for the sake of brief description. The one-third windings and remaining two-thirds windings of the slots are most commonly placed in series. FIG. 3 shows current phasor of each of the slots when a three-phase alternating current is applied to the windings in each of the slots. Currents in a direction of a 180-degree turn from currents passing through each of the terminals U. V, and W are represented by X, Y, and Z respectively. Marks (1), (2) etc. are the slot numbers. From FIG. 2 it can be seen that a U-phase current, for example, passes through the slot 1 and 2, and X-phase currents being of opposite phase to U-phase passes through the slot 7 and 8, and amplitude thereof is RR.
A winding method shown in a winding diagram of FIG. 4 is designated as short-pitch winding for the purpose of distributing current in each of the slots in a direction of rotor rotation, the distribution of which become more sinusoidal when a three-phase alternating current is applied and variations in the rotational direction of the rotor change more smoothly. To be more specific, windings in each of the slots are divided into two against the slots and half of the windings on each side are shifted to a counterclockwise direction CCW by 1-slot pitch. Solid and broken lines in FIG. 5 indicate current phasor of each of the slots when three-phase sinusoidal current is applied through providing sinusoidal current control to the current of the winding in each of the slots. For example, amplitude RS of current phasor of the windings in the slot 2 is the phasor sum UZS of U/2 and Z/2, and COS30.degree.=0.866 as compared with the amplitude RR. Amplitude SS is equal to one half of the amplitude RR.
FIG. 18 shows an example of a conventional three-phase six-pole synchronous reluctance motor. A stator 12 corresponds to the stator of FIG. 17. Nine narrow magnetic paths 14 are placed at each magnetic pole and slits which interfere with conduction of magnetic flux are placed between each of the narrow magnetic paths 14 on the rotor 13. Jointing parts for a radial direction 15 hold each of the narrow magnetic paths 14 from a portion of the center of the rotor to prevent each of the narrow magnetic paths from being broken and divided by centrifugal force during rotation of the rotor. Jointing parts for a rotor perimeter 16 are placed on a part around the rotor perimeter to link the rotor perimeter. Such a synchronous reluctance motor operates as follows. A current magnetizing magnetic flux of the stator, a d-axis current, generates magnetic flux on the narrow magnetic paths 14 in band shapes in a direction of the pass. By applying a stator torque current, that is a q-axis current, to a portion of the rotor surface, where magnetic flux is collected, in a direction of rotor rotation pointed by magnetic poles thereof, rotational torque is generated according to Fleming's left-hand rule.
FIG. 19 shows an example of a conventional three-phase six-pole induction motor. A stator 12 is the same as the stator of FIGS. 17 and 18. Rotor slots 18 for arranging a secondary electric conductor are placed in the proximity of the rotor perimeter. Various shapes such as a shape that a side of the rotor surface is open, may be applied to the rotor slots 18.
FIG. 20 shows a permanent magnet synchronous motor comprising teeth of protruding poles and windings. The detail of the motor is described in Japanese Patent Application No. Hei 10-30218. Numeral 22 indicates a stator looped by three-phase alternating-current windings. The U-phase windings MU1 and MU2 are looped through the tooth STU and the tooth STX, while the V-phase windings MV1 and MV2 are looped through the tooth STV, and the tooth STY, respectively. The W-phase windings MW1 and MW2 are looped through respective teeth STW and STZ. The width of each of the teeth is 45.degree. in terms of rotational direction degrees of the rotor and 180.degree. in terms of electrical degrees. A magnetic path bypass BPT for passing magnetic flux from the rotor to a yoke portion of the stator 22 is placed on each space between the teeth. The width of each magnetic path bypass BPT is 15.degree. and 60.degree. in terms of electrical degrees.
The relative phase of each of the of U phase, V phase, and W phase windings is 120.degree., in terms of relative electrical degrees.
Numeral 21 indicates a rotor the perimeter of which is mounted with permanent magnets 20. Magnetic poles of the permanent magnets 20 are oriented in a direction indicated by N and S of FIG. 20. In the example of the figure, the rotor has eight poles and the width of each of the magnetic poles is 45.degree. and 180.degree. in terms of electrical degrees.
The present invention was created to resolve the common problem of torque ripple.
Common denominators among the stators of conventional motors such as the motors shown in FIGS. 17, 18, 19, and 20 are as follows. Because the windings of the stator are scattered in each of the slots, a distribution of the windings is discrete in the direction of rotor rotation. Current to be applied is generally two-phase or three-phase current, discontinuous, and discrete as shown in the winding diagram of FIG. 2 and the current-phasor diagram of each of the currents of FIG. 3. Although it is basic construction for the motors of this type that the slots are scattered on the circumferences of the stator, the structure is capable of being improved through increasing the number of the slots so as to be more continuous. In addition, there is a method of skewing the stator against the rotor relatively by a 1-slot pitch for reducing the torque ripple caused by discreteness of the slots. However the method has disadvantages that complicated construction for skewing brings about an increase in motor costs and the skewing makes the output torque of the motor decrease. In the particular case of the reluctance motor shown in FIG. 18, problems that magnetic flux within the rotor operated exists in an axial direction and in the direction of rotor rotation by the skewing and components of torque ripple has been experimentally demonstrated, though such torque ripple can not be assumed from the cross-sectional view of the motor of FIG. 18.
For distribution on the circumference of the stator of currents passing through each of the slots when three-phase sinusoidal current is applied to each phase of the motor, it is ideal for the distribution of the currents to have a sinusoidal shape. However, because the same U-phase current is passing through the slot 1 and 2, for example, and a Z-phase current which is negative phase of W-phase is passing through the next slot 3 as shown in the current-phasor diagram of FIG. 3, there is a phase difference of 60.degree. in terms of electrical degrees between slot 2 and slot 3. Generated torque of a motor becomes nonuniform, even though a motor current is activated with the three-phase sinusoidal current because the current of the slots is not distributed in the sinusoidal shape as mentioned above. The result is the generation of torque ripple.
A motor having a construction as indicated by the winding diagram of the short-pitch winding of FIG. 4, wherein the current distribution on the circumference of the stator spreads in the sinusoidal shape, is considered below. It can be seen from the current-phasor diagram of FIG. 5 that an appropriate phase and suitable amplitude RR are applied to the slots 1, 3, 5, 7, 9, and 11 as described before. However, for the current applied to the slots 2, 4, 6, 8, 10, and 12, the phase is suitable but the amplitude is indicated by RS being smaller value of COS30.degree.=0.866 with respect to the amplitude RR. Thus, this motor has a problem that torque ripple is caused by this inhomogeneity.
There are further problems of magnetic vibration and magnetic noise of the motor in addition to the torque ripple when the current distribution on each of the slots is nonuniform. These problems become major factors affecting the use of such a motor in precision machines where concern for vibration and noise is great, and in household electrical appliances used in a living environment.
In the teeth of the protruding poles and windings shown in FIG. 20, winding work of the stator is easier and costs are lower as compared to the permanent magnet synchronous motor of FIG. 17. Moreover, in such a motor it is possible to loop the windings in high density and thereby minimize coil ends. This provides advantages of making the motor smaller in size and lower in cost. However, it is difficult to achieve the distribution of magnetic flux spread in the sinusoidal shape because of the simple shape of the stator. Therefore, the motor has a problem of high torque ripple. As a remedy for decreasing the torque ripple, it may be considered to alter the shape of the motor in order to be controlled by five-phase sinusoidal current. Whatever the case may be, there are disadvantages that the cost of the motor becomes high and the generated torque of motor decreases.
In addition, the high torque ripple causes problems about precision for motor control, vibrations, and noise.